1. Field of the Invention
This invention relates to the field of variable reluctance (VR) electric motors, also called stepper motors, for high torque applications.
2. Description of the Prior Art
Variable reluctance motors (VR motors) are typically used as stepper motors because they can produce rotation in small, discrete steps. This mode of operation is inherent in the nature of VR motors. VR motors have a multi-pole rotor, with the separation between poles on the stator, the pitch, different from that on the rotor. The stator poles are electromagnetically excited in separate groups or phases and the rotor rotates until its poles reach a position of minimum magnetic reluctance relative to the excited stator poles. Upon energizing successive stator phases, the rotor turns a distance equal to the rotor pitch minus the stator pitch.
Other characteristics of variable reluctance motors including their low cost, small size and high torque-to-inertia ratio make VR motors attractive for use as general purpose servomotors. Their brushless construction makes VR motors particularly suitable for applications requiring spark-free operation.
However, two drawbacks have limited the use of variable reluctance motors as servomotors: torque ripple and a nonlinear torque to input current ratio (T/I). Torque ripple is the variation in maximum available output torque as the position of the rotor poles varies with respect to the stator poles. The nonlinear T/I ratio is inherent in the design of typical VR motors because they have no permanent magnets. Torque is created by the interaction of two magnetic fields, the rotor field and the stator field, both a function of current.
In the past, in order to optimize the torque characteristics of VR motors, the stator has been the determinative element in designing the motor. Stator design balanced magnetic flux leakage, caused by having too many teeth too close together, against minimum holding torque at the stable detent position, caused by having too few teeth. The stator was designed with many teeth of uniform cross section, to provide the maximum practical area at the tips of the teeth for the magnetic flux, while maintaining sufficient intertooth space for the winding coils. The ratio of the width of the stator teeth to the width of the gap between the teeth, called the stator tooth ratio, was typically 1.0 or more. The rotor design was dependent on the stator design, with the number and width of the rotor teeth chosen to suit the geometry of the stator. The ratio of the width of the rotor teeth to the width of the gap between the teeth, called the rotor tooth ratio, was about 0.5.
One improvement on the above-described VR motors is disclosed by Konecny in U.S. Pat. No. 4,647,802, filed June 13, 1985 and issued Mar. 13, 1987.
Konecny discloses as shown in FIG. 1 making the rotor 32 rather than the stator 30 the determinative element in the design and incorporating a tapered stator tooth 31 configuration. Rotor tooth 39 is untapered. The rotor 32 is mounted on shaft 33. The rotor tooth width ratio rather than the stator tooth ratio is the basis for optimizing torque characteristics, resulting in a larger rotor tooth ratio than in the prior art of about 0.78 and a smaller stator tooth ratio than in the prior art of about 0.5. The stator 30 has fewer teeth than the rotor 32, and the stator teeth 31 are tapered so they are wider at the base 37 than at the tip 31. Konecny uses just one stator tooth 31 per pole.
Another improvement is disclosed by Gordon et al. in U.S. Pat. No. 4,496,886, filed Nov. 8, 1982 and issued Jan. 29, 1985. Gordon et al. disclose a three-state driver for the stator winding of a variable reluctance motor. The driver allows a desired current level to be achieved with only a minimum of current ripple.